1. Field of the Invention
The present invention generally relates to bipolar micro-power rail-to-rail amplifiers, and more specifically to a method for controlling a micro-power output stage so that it is capable of delivering a high ratio of load current to no load idle current.
2. Description of the Related Art
Battery powered amplifiers may spend long periods of time in an "idle" condition with no input signal, or with intermittent input signals as in the case of voice signals. Power dissipated in these idle periods is wasted and may shorten the lifetime of the battery.
The output stage of a rail-to-rail amplifier is designed to deliver a specified amount of signal power to a load and to consume low power during the idle periods. A standard measure for an amplifier's power consumption is its no load idle current. Present bipolar output stages contribute at least 100 .mu.A of no load idle current and require a relatively large number of components to deliver the desired signal gain, suitably an open loop gain of one million and a closed loop gain of one. The amount of power dissipated during idle periods is too high for applications such as battery powered instrumentation and telecommunication equipment. Furthermore, the number of components and heat dissipation requirements increase the physical dimensions of the integrated circuit, which increases its cost.
Analog Devices, Inc. produces a "Precision Rail-to-Rail Input & Output Operational Amplifier" OP284 that has the ability to swing rail-to-rail at both the input and output. The OP284 has a low supply current of approximately 800.mu. per amplifier at room temperature. The OP284's output stage uses a constant current source to supply the gain current needed to achieve sufficient signal gain, and hence idles at a relatively high current level.
Analog Devices, Inc. also produces a "Micropower Single-Supply Rail-to-Rail Input/Output Op Amp" OP291. The OP291 has a low supply current of 300.mu. per amplifier and consumes a quiescent current of 600.mu.. The OP291's output stage uses output transistor that have 180x relative emitter sizes to achieve the desired gain. This requires a lot of space on the chip. Furthermore, the output stage requires a high number of components.